This invention relates generally to recovering coordinate system information for geographic data files for which such information is lost or otherwise unavailable.
A geographic information system (GIS) generally refers to a digital representation of geographically referenced information structured to support spatial analysis. While having many purposes, GIS data files have been found to be particularly useful in electronic map production. GIS data are formatted into a set of parameters that define how the data are displayed or interpreted as a map. These parameters, which typically include projection (including various subfactors, such as parallels (meridians), spheroid and zones, depending on the projection), position on screen, scale, units and angle of rotation, are collectively referred to as a “coordinate system.” A coordinate system can contain all or part of this set of parameters.
A projection is a vital part of displaying geographic data. “Projection” is a cartographic term for a mechanism (typically a mathematical algorithm) of portraying the spherical surface of the earth, or a portion thereof, on a flat surface. In displaying the earth on a flat surface some distortions of conformality, distance, direction, scale and area always result from the process; no projection is perfect. Different projections allow mapmakers to balance these various distortions, favoring one truth over another as best suited for a particular application. Every projection has advantages and disadvantages; for instance, some are better for depicting small areas while others are better for depicting large areas. There are hundreds of different projections in use today, each with its own unique formula and set of distortions.
Modern maps are commonly produced using geographic or GIS data files that have been obtained from photographs, particularly aerial and satellite imagery. Generally, a photograph is scanned to produce a digital raster image. In a process called orthorectification, errors introduced by the photographic methodology and the three dimensional nature of real world terrain are removed from raw image data. The raster image is georeferenced by applying coordinate projection information and tying the image to the earth's surface. This coordinate system and ground tie point information, termed metadata, is represented in data files that, in many cases, are distinct or separate from the image file but can also be contained within the file itself (ex. geotiff). Such metadata can easily become damaged, corrupted, separated or lost from the data file, or otherwise unusable. Many GIS projects use raster imagery that once was properly georeferenced, but lacks sufficient accompanying metadata to tell the GIS user what the correct coordinate system is. This information is critically important for creating accurate maps from raster imagery.